by increasing and decreasing the air pressure.
The air compressor governor should cut in when a lower air pressure is reached and cut out when a higher air pressure is reached. (Check your manufacturer's specifications for these pressures.) To test this, run the engine at a fast idle to raise the air pressure until the air compressor governor cuts out. Then keep stepping on and off the brake pedal to reduce the air pressure until the air compressor governor cuts in.
B performing a visual check.
C by using the air brakes and monitoring the pressure gauges while traveling less than 40 mph.